Fixed position optical scanners are used for a number of different purposes. The most widely known use for such scanners is to detect bar code labels affixed to products sold by supermarkets or other food stores. Additional uses for scanners include detection of bar codes on parts or packages being transported by conveyors through distribution centers, warehouses or manufacturing facilities.
In supermarkets, the bar code information is used primarily to generate customer receipt tapes and secondarily, to track sales of particular items or to control inventory levels. In distribution centers, warehouses and manufacturing facilities, the bar code information is used primarily to control routing of the parts and packages.
Fixed position optical scanners typically include a laser, a beam deflecting component for deflecting a laser beam along different scan lines to produce a scanning pattern, a photodetector for sensing optical energy reflected from an item in the path of a scanning beam and a processor for extracting bar code information from the signals produced by the photodetector. The scanners also include a lockout circuit interposed between the photodetector and the processor. The function of the lockout circuit is to monitor the photodetector signals and to pass those signals only if the peak to peak voltages exceed a predetermined magnitude. If the photodetector signals meet or exceed the standards, they are passed on to the processor for decoding, normally after being converted from analog to digital form in an analog to digital converter circuit. Signals that don't exhibit the necessary spread between peak voltages are assumed to be noise voltages produced when ambient light reaches the photodetector or when the scanning beam is reflected from something other than a bar code. To prevent the system processor from being overloaded with useless noise signals, the lockout circuit inhibits or locks out signals failing to meet minimum standards.
A known type of prior art lockout circuit includes a black peak follower circuit which provides an output voltage which generally follows black peak (minimum) voltage produced by the photodetector. The prior art circuit also includes a white peak follower circuit which follows the white peak or maximum voltage produced by the photodetector. To determine whether an adequate separation between the white peak and black peak (maximum and minimum) signal voltages exist, the white peak voltage is shifted down in a voltage divider circuit and compared to the black peak voltage in a comparator amplifier. If the separation is great enough, the scaled white peak voltage will still be greater than the black peak voltage, causing the comparator amplifier to provide an output signal which will enable the photodetector signals to be passed on to the system processor.
A relatively recent development in fixed position optical scanners is the use of a rotating holographic optical element or disk to both deflect and focus a laser beam. A holographic disk usually consists of a transparent glass or plastic disk which supports a ring or annulus of holographic optical elements or facets. Each of the holographic elements or facets occupies a sector of the ring.
Each of the sectors may be generated using known off-axis holographic techniques. Depending upon the configuration of light beams used in generating a sector or facet, that facet will deflect an incident laser beam along a specific scanning path while focussing it at a specific distance from the facet surface. By changing the beam configurations used in producing different facets, a holographic disk can be produced in which different scanning beams have different focal lengths. By using some facets to produce scanning beams with shorter focal lengths and other facets to produce scanning beams with longer focal lengths, the range of distances over which at least one scanning beam will be sufficiently focussed to read a bar code label will be greatly increased.
The greater depth of field possible with a holographic disk makes the scanner much more versatile and better suited for use in materials distribution or manufacturing environments. In such environments, the bar code label to be read may be near or far from the fixed position scanner, depending upon the structure of the item on which the label appears. The ability to scan bar codes over a greater range of distances is not, however, free of problems. It is well known that the strength of an optical signal is inversely related to the square of the distance between the source of the signal and the detector. For example, if a first detector were located one meter from an optical source and a second detector were located four meters from the same source, the optical energy which reaches the second source would be equal to 1/4.sup.2 or 1/16th of the optical energy reaching the first detector. As a result, the peak to peak differential for a bar code read by a facet with a long focal length will be considerably less than the peak to peak differential for a bar code read by a facet with a short focal length.
The level of noise voltages in an optical scanner is largely independent of the focal length of the currently active facet, although the noise voltages produced when a white surface is scanned with a beam with a short focal length may be somewhat higher than normal due to "paper noise" voltages. These are random voltages which are generated when the textured paper surface is scanned by a beam which has a cross section that is relatively small in comparison to the texture. The paper fibers scatter the beam at random to produce a signal with a relatively high amount of noise.
Prior art lockout circuits of the type described above do not perform adequately where the scanner has a large depth of field. If the lockout level is set high to lock out the relatively high "paper noise" voltages produced as a result of scans with short focal length facets, the lockout level may block the smaller data signals that result from scans with long focal length facets. On the other hand, if the lockout level is set low to accommodate the relatively small data signals resulting from scans with long focal length facets, the low lockout will fail to prevent the passage of "paper noise" signals.
The variations in returned signal strength can, to some extent, be compensated for by adjusting the angular width of the individual facets of the holographic optical element. Within certain limits, the angular width of a given facet can be varied as a function of the focal length of that facet. A larger facet would collect more of the light reflected from an object than a narrower facet associated with a scanning line having a short focal length.
Using facets with different angular widths can reduce the variations in levels of returned optical energy, which reduces the problems with prior art lockout circuits. It cannot, however, provide complete compensation since the angular widths of the facets can be altered only within certain limits. Each facet must have a predetermined minimum width in order to produce a scan line of a given length.